Mammalian cells containing a nucleic acid that encodes a recombinant protein are often used to produce therapeutically or commercially important proteins. The resulting recombinant protein product contains a variety of protein molecules, each having its own set of biophysical properties. For example, a recombinant protein product produced by a mammalian cell culture can have one or more protein subpopulations, with each subpopulation having its own isoelectric point. The isoelectric point of a protein subpopulation can effect one or more of the proteins' net charge, solubility, folding, substrate binding affinity, biological activity, and clearance rate in a human body. In view of the effect of the isoelectric point on proteins, it can be desirable to shift or redistribute the subpopulations within the isoelectric profile of a recombinant protein product. For example, it may be desirable to increase or decrease a level (or quantity) of one or more of the protein subpopulation(s) having a specific isoelectric point in a recombinant protein product relative to a reference or starting isoelectric profile.